This invention concerns screens for use as filters in vibratory filtration equipment such as shakers that are used in the oil drilling industry for separating solids from the liquid phase of oil and water based muds retrieved from drilling operations. In particular the invention is concerned with the construction and manufacture of a wireframe reinforcement for embedding in a moulded support frame over which filtering material such as woven wire cloth is to be stretched and secured to form the screen.
Examples of filtering machines in which such screens are used are contained in UK 2237521 and UK 2229771.
One such screen is described in PCT/GB95/00411 (W095/23655).
The earlier design of screen extends the life of a screen by providing a sacrificial support cloth of woven wire below an upper woven wire cloth of harder wearing material than that of at least the surface of the wire from which the lower cloth is woven, so that wear due to rubbing and vibration during use occurs to a greater extent in the lower cloth than in the upper cloth. The specification also describes an improved design of frame across which woven wire cloths can be tensioned and bonded by adhesive, to form a sifting screen, in which the frame is proposed to be formed from glass reinforced gas blown polypropylene with elongate metal reinforcing elements or rods buried in the GRP. The improved frame construction is shown in FIGS. 3 to 8 of the earlier specification.
A moulded polymer support frame over which woven wire cloth is to be stretched and secured to form a sifting screen is constructed by the steps of locating in a mould tool a wire frame assembly comprising two parallel spaced apart arrays of reinforcing wires, closing the tool, injecting liquid polymer so as to wholly encapsulate the wire frame and to form an article having an open central region criss-crossed by intersecting orthogonal ribs bounded on all sides by a rigid flange, in which each of the ribs includes two parallel spaced apart wires of the said wire frame assembly, permitting the polymer to cure, opening the tool, and removing the moulded article. By arranging for two parallel spaced apart wires to extend through each of the ribs, one near one edge and the other nearer to the opposite edge of the rigs, each rib has the stiffness of a beam, and the resulting frame has high rigidity and resistance to bending, yet remains relatively lightweight.
Such a moulded support frame will be referred to as a support frame of the type described.
It is an object of the present invention to provide a method of constructing a wireframe reinforcement for such a support.
According to the present invention there is provided a method of making a reinforcing wire frame for embedding in a moulded polymer support frame over which woven wire cloth is to be stretched and secured to form a sifting screen, comprising the steps of:
(a) locating lengths of cut wire in a jig to form a first array, said lengths being substantially equally spaced apart in the jig;
(b) locating a second array of lengths of cut wire over the first array, the lengths of the second array being substantially at right angles to the lengths of wire of the first array;
(c) resistance welding the lengths of wire of the first and second arrays at points of mutual intersection so as to form a first substantially rectilinear matrix;
(d) locating lengths of cut wire in a jig to form a third array, said lengths being substantially equally spaced apart in the jig;
(e) locating a fourth array of lengths of cut wire over the third array, the lengths of the fourth array being substantially at right angles to the lengths of wire of the third array;
(f) resistance welding the lengths of wire of the third and fourth arrays at points of mutual intersection so as to form a second substantially rectilinear matrix;
(g) bending in a press break protruding lengths of wire on at least two sides of the four sides of one of the rectilinear matrices, so as to bend each protruding length first in a generally upward sense and then at a point nearer its end in a generally downward sense so that an end region of each protruding length extends parallel to a plane of the matrix but is displaced therefrom; and thereafter
(h) resistance welding the displaced ends of the protruding lengths of wire of said one matrix to protruding lengths of wire of the other matrix to form the frame.
Preferably the wire frame is selected so as to impart sufficient structural rigidity to the support frame as to prevent deflection thereof and consequent changes in the tension in the wire cloth when fitted thereto.
Additionally the wire frame is selected so as to impart sufficient strength to the support frame as to allow the latter to withstand shear stresses introduced as the frame is clamped into a vibratory screening machine.
Typically the wire frame is formed from high tensile straightened steel wire, bent as required, and in a preferred arrangement the wire is of 2.5 mm diameter.
The welding may be in part effected through the intermediary of transversely extending filler wires, so that intersections are provided where welds are to be formed between parallel protruding ends of the reinforcing wires, and the filler wires facilitate the resistance welding of the parallel protruding ends.
Preferably the wire frame fabrication is assembled so that each matrix is bowed in an outward sense, opposite to the other.
During moulding it has been found that the pressure within the mould tool can distort the framework so that the outward bowing of the opposite faces of the framework can be replaced by significant inward bendingxe2x80x94so destroying the alignment of the long rods within the upper and lower edges of the interstices of the moulded frame. To avoid this it is proposed that at least one spacer is located within the framework, so that if there is any tendency for the rod arrays to collapse inwards, the spacer will present this collapse occurring.
Preferably a plurality of spacers are located within the wire frame fabrication, each attached to one or other of the matrices so as to extend towards the other, whereby any tendency for the matrices to collapse inwards during moulding, is resisted by the spacers.
In one embodiment each spacer comprises a length of wire bent to form a shallow U with its two ends bent outwards to form two in-line lugs by which it can be welded to the underside of one of the wires which form one of the matrices, with the crest of the U section in close proximity to one of the wires of the other matrix, whereby the spacer will maintain a given dimension between the two matrices if the fabrication is subjected to a collapsing force during moulding, so causing the crest to engage the said wire of the other matrix.
In a method of moulding a support frame of the type described around a wire frame fabrication constructed in accordance with the invention, an inward force is exerted on opposite faces of the fabrication within a mould tool by fingers protruding inwardly from the inside faces of the tool, to externally engage the opposite matrices of the fabrication when the tooling closes.
The fingers preferably sandwich the fabrication in position and produce just the required inward movement of the two oppositely bowed matrices to render them parallel and spaced apart by the desired distance.
Typically the fingers comprise inwardly projecting pegs which align with crossing points of wires in the upper and lower reinforcing matrices, to space the matrices from the corresponding upper and lower internal surfaces of the mould tool and ensure that the matrices are buried within the plastics material which is injected into the mould tool during the manufacturing process.
Preferably the ends of the pegs taper to an edge, or a point.
After the mould tool is opened and the protruding pegs disengage from the struts, openings are left in the polymer. Preferably therefore the method further comprises plugging the openings with plastics material or filler.
Typically the wire frame fabrication is supported within the tooling by means of retractable pins which protrude through the tooling wall to engage the fabrication and accurately locate it within the tooling.
Conveniently the pins align with protruding ends of wires making up the fabrication and are separably joined to the ends of the wires by means of sleeves of plastics material opposite ends of which receive the pins and the reinforcing wire ends respectively.
Prior to moulding the tool may be fitted with pegs formed from a plastics material which is compatible with or is the same as the polymer material which is to be injected into the mould to encapsulate the wire frame fabrication, and the pegs become integrally bonded therein during moulding so that when the tool is opened, the pegs separate from the tool, and remain in the frame.
Protruding portions of each peg may be removed by grinding or filing or cutting.
The invention thus provides a support frame of polymer material of the type described having wire reinforcing therein constructed in accordance with the present invention, and to which woven wirecloth is to be bonded to form a filtering screen.
A filtering screen is formed using a support frame of polymer material of the type described with wire reinforcing therein constructed in accordance with the present invention, and having at least one woven wirecloth tensioned thereover and bonded to one surface thereof.
It is of course necessary to select a plastics material which is suitable to serve as a bonding medium for woven wire cloth and it has been found that polypropylene and polyethylene are suitable plastics materials although the invention is not limited to the use of polypropylene and polyethylene.
Whether polyethylene or polypropylene is used, it has been found advantageous for either material to be gas blown and glass fibre reinforced.
According to another aspect of the invention, a wire frame reinforcing fabrication for a support frame of the type described, is formed from resistance welded steel wire matrices arranged in two parallel spaced apart planes and themselves welded together along at least two edge regions by welds between protruding ends of the wires of the two matrices, so that a pair of parallel spaced apart wires will extend through each strut forming an intersecting array of struts in the support frame.
In a support frame of the type described, the reinforcing fabrication is constructed from two similar rectilinear arrays of resistance welded wires, formed by bending protruding wires along at least two sides of one of the matrices and welding the ends of the bent portions of the wires of the one matrix to the protruding ends of the wires in the other matrix, so as to maintain separation between the two matrices.
In a support frame of the type described which is to have woven wirecloth bonded to the one face thereof by locally heating the frame material and forcing the wirecloth wires into the softened material before it is allowed to cool and harden again, and which is formed from glass reinforced plastics material having embedded therein a reinforcing wire frame constructed as aforesaid, preferably the thickness of the plastics material between the reinforcing wires and the face of the support frame members to which the woven wirecloth is to be bonded is selected so as to be sufficient to enable the wirecloth to be embedded therein without making contact with the reinforcing wires.
Preferably the plastics material comprises a polypropylene or a polyethylene, and may be gas blown and glass fibre reinforced.
In a support frame of the type described, the struts will intersect similar struts which extend at right angles and a second assembly of parallel reinforcing wires is provided, running perpendicular to the first assembly of wires in planes proximate those containing the first said assembly, so that a pair of parallel spaced apart wires extends through each of the struts.
In a wire reinforcement for a support frame of the type described each reinforcing wire which is to align with the peripheral flange of the frame preferably extends into the flange at each end thereof, thereby to increase the rigidity of the flange.
Preferably the ends of the other wire of each pair are bent so that they will also become aligned with the flange of the frame, and the bent ends thereof extend into the said flanges close to the ends of the first mentioned wire to further assist in reinforcing the said flanges.
Typically the wires touch at all points of intersection and are preferably welded at all such points.
Preferably the ends of each pair of wires are welded where they occupy the flange.
In a wireframe assembly for a support frame as aforesaid, cross-point engagement may be introduced between parallel wires in the flange by incorporating intermediate transversely extending filler wire, or weld wire, between the wire ends.
Further reinforcing may be provided in the flange by means of additional reinforcing wires extending parallel to the length direction of the flange so as to overlie or underlie the protruding reinforcing wires entering the flange from the struts, and the additional reinforcing wires are welded to the protruding reinforcing wire ends.
Filtering machines of the type into which screens as aforesaid can be fitted, are described in UK Patent Specifications Nos. 2237521 and 2299771, but these are intended as examples only and the invention is not limited to the use of these screens in such machines.